The present invention relates to a method and apparatus for controlling biomass growth in a fluidized bed reactor during anaerobic or aerobic treatment of a waste feed. In particular, the present invention provides a means for concentrating a portion of bioparticles, the bioparticles being media with biomass adhered thereto and the concentrating means being integral to the fluid bed reactor, so that bioparticles delivered to a separator means for separating the biomass from the media are of high concentration requiring little downstream processing of the biomass generated therefrom.
The phenomenon of biomass growth in reactors is well known. Moreover, numerous solutions have been proposed for removing the excess biomass growth from the reactor for treatment thereof, e.g., U.S. Pat. Nos. 4,177,144 and 4,419,243, and British Patent Appl. No. 7,923,908 (filed July 9, 1979). Thus, it has been recognized that biomass growth within an anaerobic or aerobic biological fluid bed reactor must be controlled to prevent undesirable fluid bed expansion within the reactor.
In a biological expanded or fluidized bed reactor particular material (hereinafter referred to as "media") is used, such as sand, to provide a surface for biomass attachment so that the biomass may be fluidized in the reactor to interact with a waste feed to convert the waste feed into a more ecologically desirable composition. Over time, however, the biomass thickness on the media increases and the fluid bed expands. It has been found necessary to control the expansion of the fluidized bed at a certain pre-determined maximum level in order to prevent carry-over of biomass and media (hereinafter referred to as "bioparticle").
For example, British Patent Appl. No. 7,923,908 controls biomass growth by removing a fraction of the bioparticles from the top of the fluidized bed, subjecting them to a shear cell wherein supernatant liquid is removed therefrom via conduit 6 and the sheared media/biomass slurry falls into a feed hopper which can raise the slurry to a single deck vibrating sieve such that the media is returned to the fluidized bed and the biomass is exhausted as waste sludge. While satisfactory, this method of media/biomass separation results in a stream of waste sludge of relatively low concentrations of biomass often necessitating further downstream processing to allow ultimate disposal of a more concentrated byproduct. This waste sludge contains substantial amounts of waste feed liquid which has a very distasteful odor and which must be removed from the waste sludge before disposal of the excess biomass.
U.S. Pat. No. 4,177,144 discloses a controlled system to prevent the accumulation of excessive biomass in a fluidized bed reactor wherein the biomass on the media continues to grow, causing the bed to expand until its level reaches an alarming point indicative to excess growth. This point is sensed thereby activating an agitator arrangement which effects shearing of the excess biomass from the media to produce in a separator column, the column extending between the fluidized bed and the liquid effluent above the bed, a mixture of sheared material and partially stripped media. In accordance with U.S. Pat. No. 4,177,144 the media which have been sheared to remove excess biomass allegedly fall back into the bed whereas the biomass along with liquid feed is drawn off at a port provided in the separator column. This is also undesirable since the concentration of biomass being exhausted through the port of the separator is low in concentration requiring additional downstream treatment.
It has also been proposed, as shown in U.S. Pat. No. 4,419,243, that biomass growth can be controlled by providing large support structures for growing the biomass thereon whereby excess growth is prevented by removal of excess sludge from the surface of the support structure due to contact of the support structure with the internal walls of the reactor. The deficiencies of a system such as that disclosed in U.S. Pat. No. 4,419,243 are readily apparent in that the sludge is not removed from the reactor and continues to grow therein and thus results in an expanded bed.
Thus, the present inventors have come up with a method and apparatus for controlling biomass growth in a fluidized bed reactor or any anaerobic and aerobic reactor which provides an exhaust sludge high in biomass concentration and which overcomes the need for further downstream treatment of excess biomass and other deficiencies resulting therefrom.
This and other advantages of the present invention will become clear as described below.